2 Answers

Create a <gazebo> tag which references the URDF link and contains a <collision name="..."> with a name matching the generated SDF collision. In simple cases the name is the link's name + _collision. However, this isn't always the case (fixed joint lumping changes the collision name) so I recommend loading the URDF into gazebo and using the GUI left panel to figure out the actual name of the generated SDF collision.

Here is an example of torsional friction on a 1x1x1m cube defined in URDF.